Work index is defined as the

Gross energy $($kWh$/$ton of feed$)$ needed to reduce very large feed to such a size that $80\%$ of the product passes through a $100$ micron screen

Energy needed to crush one tonne of feed to $200$ microns

Energy $($kWh$/$ton of feed$)$ needed to crush small feed to such a size that $80\%$ of the product passes a $200$ mesh screen

Energy needed to crush one ton of feed to $100$ microns